Fear factor: do dugongs (Dugong dugon) trade food for safety from tiger sharks (Galeocerdo cuvier)?
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Predators can influence plants indirectly by altering spatial patterns of herbivory, so studies assessing the relationship between perceived predation risk and habitat use by herbivores may improve our understanding of community organization. In marine systems, the effects of predation danger on space use by large herbivores have received little attention, despite the possibility that predator-mediated alterations in patterns of grazing by these animals influence benthic community structure. We evaluated the relationship between habitat use by foraging dugongs (Dugong dugon) and the threat of tiger shark predation in an Australian embayment (Shark Bay) between 1997 and 2004. Dugong densities were quantified in shallow (putatively dangerous) and deep (putatively safe) habitats (seven survey zones allocated to each habitat), and predation hazard was indexed using catch rates of tiger sharks (Galeocerdo cuvier); seagrass volume provided a measure of food biomass within each zone. Overall, dugongs selected shallow habitats, where their food is concentrated. Foragers used shallow and deep habitats in proportion to food availability (input matching) when large tiger sharks were scarce and overused deep habitats when sharks were common. Furthermore, strong synchrony existed between daily measures of shark abundance and the extent to which deep habitats were overused. Thus, dugongs appear to adaptively manage their risk of death by allocating time to safe but impoverished foraging patches in proportion to the likelihood of encountering predators in profitable but more dangerous areas. This apparent food-safety trade-off has important implications for seagrass community structure in Shark Bay, as it may result in marked temporal variability in grazing pressure.
KeywordsCommunity structure Foraging Herbivory Predator intimidation Shark Bay
We thank R. Abernethy, V. Alla, L. Barre, F. Bretos, S. Buchannan, T. Bujas, J. Burghardt, S. Burghardt, C. Chow, M. Davis, H. Finn, C. Genrich, P. Green, A. Greenley, K. Harper, L. Heithaus, M. Kerford, S. Kowalewsky, A. Krickan, J. Lasky, L. Marshall, K. Martin, J. McLash, R. McPhie, B. Stalvey, J. Wilder, and K. Wirsing for field assistance. This material is based in part upon work supported by the National Science Foundation under Grant Number 0526065. Other grants and support were provided by Australian Geographic, Humminbird, Mercury Marine Australia, Monkey Mia Dolphin Resort, Monkey Mia Wildsights (Shotover), National Geographic Society Expeditions Council, National Geographic Remote Imaging, NSERC Canada grant A6869 to L.M. Dill, PADI Foundation, Shakespeare Electronics, Shark Bay Fish Factory, University of Western Australia, and public donations. We are grateful to I. Anderson, B. Barton, C. Beck, K. Crane, A. Fraser, I. Gordon, D. Rose, R. Swann, and D. Witt for logistical support. Special thanks go to D. Charles for advice, D. Capewell, and the Yadgalah Aboriginal Corporation for information about dugongs, D. Massey for extra field housing, H. Raven for temperature data, E. Elle, J. Estes, G. Hays, P. Peterson, B. Sargeant, R. Ydenberg, and anonymous reviewers for helpful comments, B. Smith, and T. Steury for statistical help, B. Black, J. Heyman, and R. Holst for their hospitality, and the Dill lab for helpful suggestions. This research was conducted under Fisheries Western Australia permits 67/97 and 08/01, Western Australia Department of Conservation and Land Management permits NE001808, SF002347, SF003818, SF004228, and SF004542, SW008085, and renewals, and Simon Fraser University Animal Care permits 639B and 653B, and complied with the current laws of the country in which it was performed. This is contribution number 27 of the Shark Bay Ecosystem Research Project.
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